linux-kernel - Re: [syzbot] UBSAN: shift-out-of-bounds in tcf_pedit

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Date:   Fri, 13 May 2022 11:36:05 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     syzbot <syzbot+8ed8fc4c57e9dcf23ca6@...kaller.appspotmail.com>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [syzbot] UBSAN: shift-out-of-bounds in tcf_pedit_init

On Thu, 2022-05-12 at 16:53 -0700, Eric Dumazet wrote:
> On Thu, May 12, 2022 at 3:51 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > 
> > On Thu, 12 May 2022 14:19:51 -0700 Eric Dumazet wrote:
> > > On Thu, May 12, 2022 at 2:18 PM syzbot
> > > > This report is generated by a bot. It may contain errors.
> > > > See https://goo.gl/tpsmEJ for more information about syzbot.
> > > > syzbot engineers can be reached at syzkaller@...glegroups.com.
> > > > 
> > > > syzbot will keep track of this issue. See:
> > > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > > > syzbot can test patches for this issue, for details see:
> > > > https://goo.gl/tpsmEJ#testing-patches
> > > 
> > > As mentioned earlier, this came with
> > > 
> > > commit 8b796475fd7882663a870456466a4fb315cc1bd6
> > > Author: Paolo Abeni <pabeni@...hat.com>
> > > Date:   Tue May 10 16:57:34 2022 +0200
> > > 
> > >     net/sched: act_pedit: really ensure the skb is writable
> > 
> > Came in as in new stack trace for an old/existing bug, right?
> > Nothing checks the shift so it'd have already tripped UBSAN
> > later on in tcf_pedit_act(), anyway.
> 
> Maybe a prior syzbot was reported, and nobody cared.
> 
> Or maybe syzbot got its way into this path only recently.

I'm reasonably sure the issue predates the bisected commit. Possibly
syzbot was unable to catch it before such commit because is much harder
to achive complete coverage of the data path, I think.

I've sent a patch, thanks for the report.

Paolo